The theory fingers symbionts as important in the origin of cooperation, sociality and eusociality and there are obvious and far-reaching implications in cultural evolution, where memes promote social interactions between hosts in order to promote their own spread during those interactions. To quote from my 2011 article on the topic:

The idea is that meme reproduction depends on social contact between humans. Increased levels of social contact between their hosts are good for memes since this results in more reproductive opportunities for them. Memes that promote human ultrasociality have the effect of pushing humans into close proximity with each other, so the memes can infect new hosts.

I'm happy to report that there's been a recent increase in the number of scientists looking into the topic, and now there's a bit more experimental evidence bearing on the issue. Some of this work is summarized in the recent popular science article: Can Microbes Encourage Altruism?. The article mostly reports on computer simulations which demonstrate the effect - which is what I was looking for in one on my 2014 articles - but the latter part of the article covers empirical evidence from a variety of sources that microbes do, in fact encourage cooperation and social behavior in their hosts - and that this can be decreased via the use of antibiotics. The article cites recent work reporting:

fruit fly larvae are attracted to airborne chemicals released by the bacteria in their guts; the appealing scent may draw the larvae toward one another

...and...

When Bienenstock exposed mice to low-dose antibiotics in utero and soon after birth, the treated mice showed lower levels of sociability and higher levels of aggression than mice in a control group

These are still early days for the hypothesis, but the topic is clearly deserving of more research.